1. Field of the Invention
This invention relates to a beam waveguide feeder for use in an aperture antenna, comprising a feed horn and a plurality of quadric surface reflectors such as revolution paraboloidal reflectors or reflectors very close to the paraboloid.
2. Description of the Prior Art
A prior art beam waveguide feeder was composed of a feed horn 1 and four reflectors 2,3,4 and 5 for example as shown in FIG. 1, in which the reflector 5 has a plane surface and the reflectors 2, 3, 4 and 5 have quadric surfaces, and are arranged in such a way as to cancel the cross polarization components generated thereon.
In an embodiment, the arrangement is a combination of a plane reflector 2 and a pair of paraboloidal reflectors each having the same focal distance and off-set angle.
With reference to FIG. 1, an explanation will be made about operation of this prior art B.W. (beam waveguide) feeder used with a Cassegrain transmission antenna.
An electric wave fed from a transceiver 12 through feed horn 1 is reflected at the four reflectors including plane reflector 2, paraboloidal reflectors 3 and 4, and plane reflector 5, and focuses at a point 8, then it travels to the Cassegrain antenna consisting of a subreflector 6 and a main-reflector 7, from which it is radiated.
The wave transmitted from the B.W. feeder is supplied to the antenna as if it originated from an assumed feed horn 1' with its phase center at the point 8 (hereinafter, this horn is called the equivalent feed horn). In such a B.W. feeder, the Cassegrain antenna 6,7 and the plane reflector 5 are revolvable about the elevation axis 11 in scanning the antenna beam about the elevation axis 11; therefore it is not necessary to move the feed horn 1.
On the other hand, it is possible to scan the antenna beam around the azimuth axis 10 by a revolution of the entire unit consisting of the antenna, plane reflector 2, paraboloidal reflectors 3 and 4 and plane reflector 5 about the azimuth axis 10. With this B.W. feeder, the feed horn 1 can stand still while the equivalent feed horn 1' is moving. This feeder makes it possible to scan the antenna beam with the feed horn 1 connected to a transceiver 12 fixed on the ground.
So far, the explanation has been made of a prior art B.W. feeder employed in a Cassegrain antenna. Next, an explanation will be made of the B.W. feeder utilized in a spherical reflector antenna. As shown in FIG. 2, the spherical reflector antenna consists of a spherical reflector 15 and a feed horn 1, and is characterized in that beam scanning is carried out by a revolution of the feed horn 1 about the center 16 of the spherical reflector 15 instead of moving the spherical reflector 15.
FIG. 3 shows an example in which the prior art B.W. feeder of FIG. 1 is applied to spherical reflector 15. In the drawing, the spherical reflector 15 is used in off-set form so as to avoid blocking of the antenna aperture surface by the B.W. feeder. To correct a factor such as spherical aberration of reflector 15, one or more sub-reflectors may be provided between the spherical reflector 15 and equivalent feed horn 1'. This is explained in detail in the paper written by the inventor of this application: Watanabe, Mizuguchi "On the Design Method for Reflector Surfaces of an Offset Spherical Reflector Antenna". Paper of Technical Group TGAP 81-29 (1981, 6,25)--Institute of Electro Communication in Japan.
The B.W. feeder comprising a feed horn 1, plane reflector 2, paraboloidal reflectors 3 and 4 and a plane reflector 5 is the same as that of FIG. 1.
By a revolution of plane reflector 5 about axis 11 which passes through the center 16 of spherical reflector 15, the beam radiated from the antenna can be deviated around the axis 11. By a revolution of a structure consisting of plane reflector 2, paraboloidal reflectors 3 and 4 and plane reflector 5 about an axis 10 which passes through the center 16 of spherical reflector 15 and the phase center 9 of the feed horn 1, the beam radiated from the antenna can be deviated about the axis 10.
By use of the above mentioned structure in the manner described, it is not necessary to move the spherical reflector 15 and the feed horn 1 in scanning the antenna radiation beam.
In the prior art apparatus of FIG. 3, the cross point of the two revolution axes 10 and 11 of the B.W. feeder must be at the center 16 of the spherical reflector 15, therefore the apparatus exhibits the following three problems:
(1) In a spherical reflector antenna, the equivalent feed horn 1' is located at a half distance of the radius R of spherical reflector 15. Therefore, the plane reflector 5 placed at the center of spherical reflector 15 must be as large as the reflector 15. Because of this restrictive condition, this type of antenna is impractical.
(2) Since the reflector 15 has a spherical aberration, the effective aperture D of the spherical reflector antenna can not be larger than the radius R of spherical reflector 15. Especially in the case of the off-set type, in practice, the radius R of the reflector 15 should be about twice the effective aperture D of the spherical reflector antenna. Accordingly, the wave transmission distance between the B.W. feeder and the antenna will be very long, thereby reducing transmission efficiency as well as requiring a huge structure.
(3) The spherical reflector antenna is useful if it is used as a multiple beam antenna provided with plural feed horns to give plural beams. The B.W. feeder of FIG. 3, however, can not accomodate plural beam guides to feed a single spherical main reflector, because the plane reflector 5 must be positioned at the center 16 of spherical reflector 15.
These problems arise because the mechanism is such that the equivalent feed horn 1' can not move beyond the revolution around the axes 10 and 11.
For such beam steerable antennas as spherical reflector antennas which can scan the beam with their main reflector fixed, torus antennas and bifocal antennas, each type antenna requires its particular equivalent horn motion. The above mentioned prior art B.W. feeder, however, is incapable of moving the equivalent feed horn to an arbitrary position, nor is it capable of directing it in arbitrary direction, and therefore it is substantially impossible to fix the feed horn.